U.S. patent number 6,579,342 [Application Number 09/778,630] was granted by the patent office on 2003-06-17 for oleophobic membrane materials by oligomer polymerization for filter venting applications.
This patent grant is currently assigned to Pall Corporation. Invention is credited to Jeff Palpallatoc, I-fan Wang.
United States Patent |
6,579,342 |
Wang , et al. |
June 17, 2003 |
Oleophobic membrane materials by oligomer polymerization for filter
venting applications
Abstract
Oleophobic and hydrophobic filters for filter venting
applications are made by forming a fluorosulfone coating on the
surface of a filtration substrate. The filters have high water
penetration pressures and high air permeabilities. The coatings are
formed by grafting a fluorosulfone oligomer to a polymeric
substrate.
Inventors: |
Wang; I-fan (San Diego, CA),
Palpallatoc; Jeff (San Diego, CA) |
Assignee: |
Pall Corporation (East Hills,
NY)
|
Family
ID: |
25113961 |
Appl.
No.: |
09/778,630 |
Filed: |
February 7, 2001 |
Current U.S.
Class: |
95/46; 55/524;
55/DIG.5; 96/13; 96/14; 96/224; 96/225; 96/6 |
Current CPC
Class: |
B01D
39/1692 (20130101); B01D 2239/0478 (20130101); B01D
2239/0613 (20130101); B01D 2239/0618 (20130101); B01D
2239/0622 (20130101); B01D 2239/0654 (20130101); B01D
2239/1216 (20130101); B01D 2239/125 (20130101); Y10S
55/05 (20130101) |
Current International
Class: |
B01D
39/20 (20060101); B01D 39/16 (20060101); B01D
019/00 (); B01D 053/22 () |
Field of
Search: |
;95/45-55 ;96/4,6,10-14
;55/524,DIG.5 ;428/98 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Tanny, et al: Microporous membrane laminate. Data supplied from the
esp@cenet database--12, Apr. 1, 1987..
|
Primary Examiner: Spitzer; Robert H.
Attorney, Agent or Firm: Knobbe, Martens, Olson & Bear,
LLP
Claims
What is claimed is:
1. A method of venting an intravenous fluid, the method comprising
the steps of: providing an intravenous fluid; providing a
receptacle, the receptacle containing the intravenous fluid, the
receptacle having a vent, the vent having an oleophobic filter
comprising a substrate and a coating, the substrate comprising a
polymer and the coating comprising a fluorosulfone oligomer capable
of being covalently bonded to the polymer, wherein the substrate is
rendered oleophobic by grafting the fluorosulfone oligomer to the
substrate; and venting a liquid or a gas through the oleophobic
filter.
2. The method of claim 1, further comprising the step of steam
sterilizing the oleophobic filter.
3. The method of claim 1, further comprising the step of
sterilizing the oleophobic filter using an ionizing radiation.
4. A medical device, the device comprising an oleophobic filter,
the oleophobic filter comprising a substrate and a coating, the
substrate comprising a polymer and the coating comprising a
fluorosulfone oligomer capable of being covalently bonded to the
polymer, wherein the substrate is rendered oleophobic by grafting
the fluorosulfone oligomer to the substrate.
5. The medical device of claim 4, wherein the oleophobic filter
comprises an intravenous fluid vent filter.
6. An oleophobic filter comprising a substrate and a coating, the
substrate comprising a polymer and the coating comprising a
fluorosulfone oligomer capable of being covalently bonded to the
polymer, wherein the substrate is rendered oleophobic by grafting
the fluorosulfone oligomer to the substrate.
7. The oleophobic filter of claim 6, wherein the polymer comprises
a polysulfone.
8. The oleophobic filter of claim 7, wherein the polysulfone is
selected from the group consisting of a polyalkylsulfone, a
polyethersulfone, and a polyarylsulfone.
9. The oleophobic filter of claim 6, wherein the polymer comprises
a polyvinylidene fluoride.
10. The oleophobic filter of claim 6, wherein the polymer is
selected from the group consisting of a polyethylene, a
poly(tetrafluoroethylene), a poly(tetrafluoroethylene-co-ethylene),
a polyamide, a polyacrylate, a polymethacrylate, a polyester, a
polypropylene, a nylon, and a polyurethane.
11. The oleophobic filter of claim 6, wherein the substrate
comprises a porous membrane.
12. The oleophobic filter of claim 11, wherein the porous membrane
comprises an isotropic membrane.
13. The oleophobic filter of claim 11, wherein the porous membrane
comprises an anisotropic membrane.
14. The oleophobic filter of claim 13, wherein the anisotropic
membrane comprises an asymmetric membrane.
15. The oleophobic filter of claim 14, wherein the asymmetric
membrane has a supporting structure, a first porous face and a
second porous face, each porous face having pore diameters, and
wherein an asymmetry between the pore diameters of the first porous
face and the second porous face is at least about 2:1.
16. The oleophobic filter of claim 15, wherein the asymmetry
between the pore diameters of the first porous face and the second
porous face is at least about 5:1.
17. The oleophobic filter of claim 15, wherein the asymmetry
between the pore diameters of the first porous face and the second
porous face is at least about 10:1.
18. The oleophobic filter of claim 15, wherein the supporting
structure comprises an isotropic region adjacent the second porous
face, the isotropic region having substantially constant pore size,
the supporting structure further comprising an asymmetric region
adjacent the isotropic region.
19. The oleophobic filter of claim 18, wherein the asymmetric
region extends through at least about 50% of the supporting
structure but not more than about 85% of the supporting
structure.
20. The oleophobic filter of claim 15, wherein the average diameter
of the pores of the second porous face is between about 0.01 .mu.m
and about 50 .mu.m.
21. The oleophobic filter of claim 15, wherein the average diameter
of the pores of the second porous face is between about 0.01 .mu.m
and about 10 .mu.m.
22. The oleophobic filter of claim 15, wherein the average diameter
of the pores of the second porous face is less than about 0.01
.mu.m.
23. The oleophobic filter of claim 6, wherein the polymeric
substrate comprises a material selected from the group consisting
of a nonwoven material, a woven material, and a melt blown
material.
24. The oleophobic filter of claim 6, wherein the fluorosulfone
oligomer comprises a polyfluorosulfone acrylate.
25. The oleophobic filter of claim 6, further comprising a support,
wherein the substrate is bonded to the support.
26. The oleophobic filter of claim 25, wherein the support
comprises a fabric.
27. The oleophobic filter of claim 25, wherein the support is
selected from the group consisting of a polysulfone, a
polyethylene, a poly(tetrafluoroethylene), a
poly(tetrafluoroethylene-co-ethylene), a polyamide, a polyacrylate,
a polymethacrylate, a polyester, a polypropylene, a nylon, and a
polyurethane.
28. The oleophobic filter of claim 6, wherein the fluorosulfone
oligomer has a structure of formula:
wherein n is an integer from about 5 to about 20, and wherein m is
an integer from about 2 to about 10.
29. A method of producing an oleophobic filter, comprising:
providing a polymeric substrate; contacting the substrate with a
grafting formulation comprising a fluorosulfone oligomer; grafting
the fluorosulfone oligomer to the substrate; and recovering an
oleophobic filter.
30. The method of claim 29, further comprising: providing a
support; and bonding the substrate to the support.
31. The method of claim 30, wherein the step of bonding the
substrate to the support is conducted prior to contacting the
substrate with the grafting formulation.
32. The method of claim 30, wherein the step of bonding the
substrate to the support is conducted after contacting the
substrate with the grafting formulation.
33. The method of claim 29, wherein the grafting formulation
comprises a solvent for the fluorosulfone oligomer.
34. The method of claim 33, wherein the solvent comprises a
non-polar solvent.
35. The method of claim 33, wherein the solvent comprises a polar
solvent.
36. The method of claim 35, wherein the solvent comprises isopropyl
alcohol.
37. The method of claim 35, wherein the solvent comprises
water.
38. The method of claim 35, wherein the solvent comprises a mixture
of isopropyl alcohol and water.
39. The method of claim 29, wherein the grafting formulation
comprises between about 0.05 and about 40 wt. % fluorosulfone
oligomer.
40. The method of claim 39, wherein the grafting formulation
comprises between about 0.1 and about 10 wt. % fluorosulfone
oligomer.
41. The method of claim 40, wherein the grafting formulation
comprises between about 0.5 and about 5 wt. % fluorosulfone
oligomer.
42. The method of claim 41, wherein the grafting formulation
comprises between about 1 and about 2 wt. % fluorosulfone
oligomer.
43. The method of claim 29, wherein the grafting step comprises
exposing the coated substrate to ultraviolet radiation.
44. The method of claim 29, further comprising rinsing the
oleophobic filter in a rinsing liquid.
45. The method of claim 44, wherein the rinsing liquid comprises
water.
46. The method of claim 44, wherein the rinsing liquid comprises
isopropyl alcohol.
47. The method of claim 44, wherein the rinsing liquid comprises a
mixture of isopropyl alcohol and water.
48. The method of claim 44, further comprising drying the
oleophobic filter at an elevated temperature, wherein the drying
step is conducted after the rinsing step.
Description
FIELD OF THE INVENTION
The present invention relates to filtration media having both
hydrophobic (water repellent) and oleophobic (oil repellent)
properties. The properties are produced by forming a fluorosulfone
oligomer coating on a substrate such as a hydrophobic or
hydrophilic membrane or other filtration medium. The invention also
relates to methods of preparing such filtration media.
BACKGROUND OF THE INVENTION
Hydrophobic filters are used in filtration of gases, in venting
filters, and as gas vents. These hydrophobic filters allow gases
and vapors to pass through the filter while liquid water is
repelled by the filter.
Polytetrafluoroethylene (PTFE) has been the most commonly used
material in filters for gas venting. PTFE is chemically and
biologically inert, has high stability, and is hydrophobic. PTFE
filters therefore allow gases to be selectively vented while being
impervious to liquid water.
Hydrophobic membranes are used as filters in healthcare and related
industries, for example, as vent filters for intravenous (IV)
fluids and other medical devices. In the healthcare industry, the
membrane is sterilized before use. PTFE membranes can be sterilized
for these health-related applications with steam or by chemical
sterilization without losing integrity.
Treating PTFE membranes with steam can cause pore blockage due to
condensation of oil from the machinery used to generate the steam.
The resulting loss of air permeability reduces the membrane's
ability to serve as an air vent. Although chemical sterilization
minimizes exposure of the membrane to oil, chemical sterilization
uses toxic chemicals and can generate byproducts which cause waste
disposal problems. Ionizing radiation has also been used for
sterilization of materials used in medical and biological devices.
PTFE may become unstable when exposed to ionizing radiation.
Irradiated PTFE membranes have greatly reduced mechanical strength
and cannot be used in applications where they are subjected to even
moderate pressures.
Perhaps the two biggest drawbacks to PTFE as a filter for venting
gases are the high cost and the low air permeability of PTFE
membranes. PTFE membranes are formed by extruding and stretching
PTFE. Both the PTFE raw material and the processing to form the
PTFE membrane are expensive. Furthermore, the extruding and
stretching processes used to form PTFE membranes create a membrane
which has relatively low air permeability.
The oleophobicity of PTFE can be improved by impregnating or
coextruding the PTFE with siloxanes (for example, U.S. Pat. No.
4,764,560), fluorinated urethane (U.S. Pat. No. 5,286,279), or
perfluoro-2,2-dimethyl-1,3-dioxole (U.S. Pat. No. 5,116,650).
Although the oil resistance of the PTFE is improved, the treated
PTFE membranes are expensive, and air permeability remains fairly
low.
As a result, efforts have been made to identify alternative
substrates which are less expensive and have higher air
permeability than PTFE and which can be modified to be hydrophobic
and oleophobic.
Coating filtration substrates allows one to retain the desirable
bulk properties of the substrate while only altering the surface
and interfacial properties of the substrate. Coating substrates to
increase their hydrophobic and oleophobic properties has not been
very practical, because the coatings can reduce permeability.
Furthermore, many of the coating materials are expensive.
Scarmoutzos (U.S. Pat. No. 5,217,802) modified the surface of
substrates made of nylon, polyvinylidene difluoride (PVDF), and
cellulose by treating the substrate with a fluorinated acrylate
monomer. The substrate was sandwiched between two sheets of
polyethylene, and the monomer was polymerized by exposing to
ultraviolet light. The resulting composite filters had hydrophobic
and oleophobic surfaces. The air permeability of the filters
decreases with time.
Moya (U.S. Pat. No. 5,554,414) formed composite filters from
polyethersulfone and PVDF membranes with a method similar to that
of Scarmoutzos. The resulting filters did not wet with water or
hexane. The disadvantage of the Moya filters is that air
permeability of the treated filters was lower than the untreated
substrates, and the fluorinated monomer is expensive.
Sugiyama et al. (U.S. Pat. No. 5,462,586) treated nylon fabric and
PTFE membranes with solutions containing two different preformed
fluoropolymers. The treated filters were resistant to water and
oils. The durability of filters coated with preformed polymers is
often less than that for filters where the coating is formed by
polymerizing a monomer on the surface of the substrate,
however.
Kenigsberg et al. (U.S. Pat. No. 5,156,780) treated a variety of
membranes and fabrics with solutions of fluoroacrylate monomers and
formed coatings on the substrate by polymerizing the monomer. The
coating conferred oil and water repellency onto the substrate.
However, the airflow through the treated membrane was reduced,
compared to the untreated membrane.
Hydrophobic media suitable for garments have been made by extruding
mixtures of polypropylene or PTFE and the fluorochemical
oxazolidinone as disclosed in U.S. Pat. No. 5,260,360. The media
made by extrusion tend to have relatively low air permeability.
In copending U.S. application Ser. No. 09/323,709 filed Jun. 1,
1999, now U.S. Pat. No. 6,355,081 issued Mar. 12, 2002
(incorporated herein by reference in its entirety), oleophobic and
hydrophobic filters are prepared by forming a polydimethylsiloxane
coating on a polymeric substrate by polymerizing vinyl terminated
siloxane with a crosslinker such as hydrosilicon in the presence of
a catalyst.
SUMMARY OF THE INVENTION
There is a need for an oleophobic and hydrophobic filter which is
inexpensive and has high air permeability. Specifically, there is a
need for a coating for filter medium substrates that makes the
substrate oleophobic and hydrophobic, and also a need for a more
cost-effective process of making oleophobic filters.
In a first embodiment of the present invention, an oleophobic
filter is provided including a substrate and a coating, the
substrate including a polymer and the coating including a
fluorosulfone oligomer capable of being covalently bonded to the
polymer, wherein the substrate is rendered oleophobic by grafting
the fluorosulfone oligomer to the substrate.
In various aspects of the first embodiment, the polymer includes a
polysulfone, for example, a polyalkylsulfone, a polyethersulfone,
and a polyarylsulfone. The polymer may also include a
polyvinylidene fluoride, a polyethylene, poly(tetrafluoroethylene),
a poly(tetrafluoroethylene-co-ethylene), a polyamide, a
polyacrylate, a polymethacrylate, a polyester, a polypropylene, a
nylon, or a polyurethane.
In another aspect of the first embodiment, the substrate includes a
porous membrane. The porous membrane may include an isotropic
membrane or anisotropic membrane, such as an asymmetric membrane.
If the substrate is an asymmetric membrane, the asymmetric membrane
may have a supporting structure, a first porous face and a second
porous face, each porous face having pore diameters, wherein an
asymmetry between the pore diameters of the first porous face and
the second porous face is at least about 2:1, at least about 5:1,
or at least about 10:1. The supporting structure may also include
an isotropic region adjacent the second porous face, the isotropic
region having substantially constant pore size, the supporting
structure further including an asymmetric region adjacent the
isotropic region. The asymmetric region may extend through at least
about 50% of the supporting structure but not more than about 85%
of the supporting structure. The average diameter of the pores of
the second porous face may be between about 0.01 .mu.m and about 50
.mu.m, between about 0.01 .mu.m and about 10 .mu.m, or may be less
than about 0.01 .mu.m.
In a further aspect of the first embodiment, the polymeric
substrate includes a material including a nonwoven material, a
woven material, or a melt blown material.
In another aspect of the first embodiment, the fluorosulfone
oligomer includes a polyfluorosulfone acrylate.
In a further aspect of the first embodiment, the oleophobic filter
further includes a support, wherein the substrate is bonded to the
support. The support may include a fabric. The support may include
a polysulfone, a polyethylene, a poly(tetrafluoroethylene), a
poly(tetrafluoroethylene-co-ethylene), a polyamide, a polyacrylate,
a polymethacrylate, a polyester, a polypropylene, a nylon, or a
polyurethane.
In another aspect of the first embodiment, the fluorosulfone
oligomer has a structure of formula C.sub.n F.sub.2n+1 SO.sub.2
N(CH.sub.2 CH.sub.2)CH.sub.2 CH.sub.2 OCO--(CH.sub.2
--CH.sub.2).sub.m --CH.dbd.CH.sub.2, wherein n is an integer from
about 5 to about 20, and wherein m is an integer from about 2 to
about 10.
In a second embodiment of the present invention, a method of
producing an oleophobic filter is provided, the method including
providing a polymeric substrate; contacting the substrate with a
grafting formulation including a fluorosulfone oligomer; grafting
the fluorosulfone oligomer to the substrate; and recovering an
oleophobic filter.
In an aspect of the second embodiment, the method further includes
providing a support and bonding the substrate to the support.
In a further aspect of the second embodiment, the method further
includes providing a support; and bonding the polymeric substrate
to the support, wherein the substrate is bonded to the support
prior to or after contacting the substrate with the grafting
formulation.
In another aspect of the second embodiment, the grafting
formulation includes a solvent for the fluorosulfone oligomer. The
solvent may include a non-polar solvent or a polar solvent, such as
isopropyl alcohol, water, or a mixture of isopropyl alcohol and
water.
In a further aspect of the second embodiment, the grafting
formulation includes between about 0.05 and about 40 wt. %, between
about 0.1 and about 10 wt. %, between about 0.5 and about 5 wt. %,
or between about 1 and about 2 wt. % fluorosulfone oligomer.
In another aspect of the second embodiment, the grafting step
includes exposing the coated substrate to ultraviolet
radiation.
In a further aspect of the second embodiment, the method may
further include rinsing the oleophobic filter in a rinsing liquid.
The rinsing liquid may include water, isopropyl alcohol, or a
mixture of isopropyl alcohol and water.
In another aspect of the second embodiment, the method further
includes drying the oleophobic filter at an elevated temperature,
wherein the drying step is conducted after the rinsing step.
In a third embodiment of the present invention, a medical device is
provided, the device comprising an oleophobic filter, the
oleophobic filter comprising a substrate and a coating, the
substrate comprising a polymer and the coating comprising a
fluorosulfone oligomer capable of being covalently bonded to the
polymer, wherein the substrate is rendered oleophobic by grafting
the fluorosulfone oligomer to the substrate.
In one aspect of the third embodiment, the oleophobic filter
includes an intravenous fluid vent filter.
In a fourth embodiment of the present invention, a method of
venting an intravenous fluid is provided, the method comprising the
steps of providing an intravenous fluid; providing a receptacle,
the receptacle containing the intravenous fluid, the receptacle
having a vent, the vent having an oleophobic filter including a
substrate and a coating, the substrate comprising a polymer and the
coating including a fluorosulfone oligomer capable of being
covalently bonded to the polymer, wherein the substrate is rendered
oleophobic by grafting the fluorosulfone oligomer to the substrate;
and venting a fluid through the oleophobic filter.
In an aspect of the fourth embodiment, the method further includes
the step of steam sterilizing the oleophobic filter.
In another aspect of the fourth embodiment, the method further
includes the step of sterilizing the oleophobic filter using an
ionizing radiation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The following description and examples illustrate a preferred
embodiment of the present invention in detail. Those of skill in
the art will recognize that there are numerous variations and
modifications of this invention that are encompassed by its scope.
Accordingly, the description of a preferred embodiment should not
be deemed to limit the scope of the present invention.
The present invention provides hydrophobic and oleophobic filters
that have high gas permeabilities and that repel water and other
liquids. The invention also includes methods of preparing such
filters.
The filter medium substrate is treated with a coating material
including a crosslinked fluorosulfone-containing oligomer, which
coats the surface of the substrate. Coating the substrate with a
material including crosslinked fluorosulfone imparts permanent
oleophobicity and hydrophobicity to the filter. The treated filters
have high permeabilities for airflow and reject liquid water, as
evidenced by high water penetration pressures. The filters are
useful, for example, as air filters or vent filters for intravenous
(IV) or other medical devices. The critical surface tension for
spreading (.gamma..sub.c), which is defined as the wettability of a
solid surface by noting the lowest surface tension a liquid can
have and still exhibit a contact angle (.theta.) greater than zero
degrees on that solid, is dramatically reduced for a substrate
after treatment according to the method of the invention.
Introduction
Crosslinking is a process wherein a low molecular weight active
group, such as an oligomer, is covalently bonded to a parent
polymer (for example, a polysulfone or PVDF) so as to modify the
surface of the polymer. In one embodiment of such a chemical
grafting process, a polymeric substrate is immersed in a solution
containing one or more oleophobic fluorosulfone oligomers and
polymerization initiators, then post-treated with UV radiation to
induce grafting of the fluorosulfone oligomer to the polymeric
substrate. In this way, permanent oleophobic groups may be
covalently bonded to a polymeric substrate.
The Polymeric Substrate
The membrane or other substrate of the filtration media of
preferred embodiments may be prepared from any suitable polymer
capable of being grafted with the fluorosulfone oligomer. The
polymer may be a homopolymer, copolymer, terpolymer, or more
complex polymer. A single polymer or combination of two or more
polymers may be preferred. The polymer may be subjected to a
pretreatment, for example, sulfonation or grafting prior to forming
a membrane casting dope, or may be subjected to a post-treatment,
for example grafting or crosslinking, after a membrane is cast or a
substrate is formed. There is no particular molecular weight range
limitation for useful polymers.
In a particularly preferred embodiment, the polymer is a sulfone
polymer, such as polysulfone, polyethersulfone (PES), or
polyarylsulfone. Other suitable polymers include fluorinated
polymers such as polyvinylidene fluoride (PVDF), polyolefins
including polyethylene and polypropylene, polytetrafluoroethylene
(PTFE or Teflon.TM.), poly(tetrafluoroethylene-co-ethylene) (ECTFE
or Halar.TM.), acrylic copolymers, polyamides or nylons,
polyesters, polyurethanes, polycarbonates, polystyrenes,
polyethylene-polyvinyl chloride, polyacrylonitrile, cellulose, and
mixtures or combinations thereof.
The substrates that may be coated may be in any suitable shape or
form. If the substrate is a membrane, suitable forms include, but
are not limited to, sheet and hollow fiber cast polymer membranes.
Suitable membranes further include both those membranes that are
cast from a single polymer solution or dope, generally referred to
as "integral" membranes, as well as non-integral or composite
membranes that are cast from more than one polymer solution or dope
to form a layered or composite membrane. Composite membranes may
also be assembled from two or more fully formed membranes after
casting, for example, by lamination or other suitable methods.
Suitable composite membranes are discussed further in copending
U.S. patent application Ser. No. 09/694,120 filed on Oct. 20, 2000
and entitled "LAMINATES OF ASYMMETRIC MEMBRANES," which is
incorporated herein by reference in its entirety. Polymeric
substrates other than membranes are also suitable for grafting with
fluorosulfone oligomer according to preferred embodiments.
Non-limiting examples of other suitable substrates include hollow
fiber media, melt blown or other nonwoven media, woven media,
extruded media, and sedimented media. Suitable melt blown
substrates include, but are not limited to, polyester,
polypropylene or ECTFE, and are commercially available from U.S.
Filter/Filterite Division, Timonium, Md.
The filtration media of preferred embodiments may be composites,
such as, for example, composites having different layers of any of
the foregoing media, composites having multiple layers of the same
medium, or composites having layers of the same medium, but of
different pore sizes, porosities, geometries, orientations, and the
like.
The substrates that are suitable for coating in accordance with the
present invention may include membranes having a symmetric or
asymmetric pore structure. The term "asymmetric" as used herein
relates to a membrane possessing a pore size gradient. That is,
asymmetric membranes possess their smallest or finest pores in or
adjacent to one surface of the membrane, generally referred to as
the "skin" surface or "shiny" side of the membrane. The increase in
pore size between the skin surface and the opposite surface of the
membrane is generally gradual, with the smallest pore size nearest
the skin surface and the largest pores being found at or adjacent
to the opposite, coarse-pored surface, generally referred to as the
"open" surface or the "dull" side of the membrane. Another variety
of asymmetric membrane, commonly described as having a
"funnel-with-a-neck" structure, includes both an asymmetric region
and an isotropic region, the isotropic region having a uniform pore
size. The isotropic region typically extends from the skin surface
of the membrane through about 5-80% of the thickness of the
membrane, more preferably from about 15-50% of the thickness of the
membrane.
The membranes of preferred embodiments also have a porous
supporting structure between the two sides of the membrane. The
nature of the porous supporting structure of a membrane may depend
on the composition of the casting dope and the coagulation bath.
The supporting structure can include closed cells, open cells of
substantially the same pore size from one side of the membrane to
the other, open cells with a gradation of pore sizes from one side
of the membrane to the other, or finger-type structures, generally
referred to as "macrovoids." Macrovoids typically vary
substantially in size from the surrounding porosity, and generally
do not communicate with surface pores. In a preferred embodiment,
the porous supporting structure includes a network of structural
surfaces capable of contacting a filter stream, defined herein as
including any fluid substance, including liquids and gases, that
passes through the membrane via the porous supporting
structure.
Whether the membrane has an isotropic, asymmetric or
funnel-with-a-neck structure can depend upon several factors
involved in the preparation of the membrane, including the type and
concentration of the polymer, the solvent, and the nonsolvent; the
casting conditions such as the knife gap, and the dope temperature;
environmental factors such as the exposure time between casting and
quenching, and the humidity of the exposure atmosphere; and the
composition and temperature of the quench bath. In various
embodiments, the asymmetry in pore size between the skin side and
dull side of the membrane may typically be from about 1:2, 1:5,
1:10, 1:20, 1:50, 1:100, or 1:200 to about 1:1,000 or 1:10,000 or
greater, more preferably from about 1:2, 1:5, 1:10, or 1:20 to
about 1:50, 1:100, 1:200 or 1:1,000.
Membranes that are suitable for grafting in accordance with the
present invention include virtually any formed hydrophobic or
hydrophilic polymer membranes. Suitable membranes may typically
have pore diameters from about 0.001 .mu.m to about 50 .mu.m or
greater, preferably from about 0.01 .mu.m to about 50 .mu.m, on the
skin side of the membrane. Membranes that are suitable for coating
in accordance with the preferred embodiments include, for example,
membranes that typically possess porosities characteristic of
microfiltration membranes. Microfiltration membranes typically
possess pore diameters of from at least about 0.01 or 0.05 .mu.m to
about 5, 8, 10 or 20 .mu.m on the skin side of the membrane.
Membranes within the ultrafiltration range may also be grafted
according to preferred embodiments. Ultrafiltration membranes
typically possess molecular weight cutoffs of from about 10,000
Daltons to about 1,000,000 Daltons and may have pore diameters
typically from about 0.001 .mu.m to about 0.050 .mu.m on the skin
side of the membrane.
Particularly preferred membranes before post treatment, such as
crosslinking or grafting, include the highly asymmetric
polyethersulfone membranes disclosed in U.S. Pat. No. 5,886,059
(incorporated herein by reference in its entirety). In typical
highly asymmetric PES membranes, one side of the PES membrane is a
skin face having relatively small diameter pores while the opposite
or dull face of the membrane has relatively large diameter pores.
The difference in porosity between the skin face and the opposite
face is typically from at least about 1:2, 1:5, or 1:10 to about
1:20, 1:50, 1:100, 1:200 or 1:10,000. Preferably, the difference in
porosity is from about 1:2 to about 1:10,000. More preferably, the
difference in porosity is from about 1:2 to about 1:200. Most
preferably, the difference in porosity is from about 1:5 to about
1:20. In addition, such membranes generally have a gradual slope of
pore size from the skin face to the opposite face. Thus, during
filtration, larger particles can enter the membrane through the
larger pores, but do not exit through the smaller pores. Because
the larger particles become lodged just within the outer surface,
the membranes made by the methods included herein are not easily
clogged with large particles.
In another preferred embodiment, the substrate is a microporous
PVDF polymer membrane having a microporous surface with minimum
pores, and an opposite surface with maximum pores. Such membranes
may be prepared from PVDF HYLAR-461, (available from Ausimont USA,
Inc. of Thorofare, N.J.) and may also typically contain from about
1% to about 30% by weight of polyvinylpyrrolidone (PVP).
Hydrophilic membranes may also be coated according to the present
invention. Such hydrophilic membranes include hydrophobic membranes
that have been post-treated with a surfactant or other material
capable of rendering the membrane hydrophilic, as well as membranes
prepared from a casting dope containing a hydrophilic material in
addition to a hydrophobic polymer.
The filtration media of the preferred embodiments may include
composite membranes. Composite membranes are membranes having
multiple layers, and are preferred in a variety of separations
applications. In many cases, the various layers of a composite
membrane each impart different desirable properties to the
composite. For example, in some applications, an extremely thin
membrane may have advantageous flow rates in separations of very
small particles, gases, and the like. Yet such a thin membrane may
be fragile and difficult to handle or to package into cartridges.
In such cases, the fragile, thin layer membrane may be combined
with a backing or with a stronger, more porous membrane, to form a
composite having improved strength and handling characteristics
without sacrificing the separation properties of the thin layer
membrane. Other desirable properties imparted by laminating a
membrane to another media may include increased burst strength,
increased thickness, providing prefiltration capability, and
providing an adhesive layer to facilitate assembly of a device.
Composite membranes may be prepared using lamination techniques. In
lamination, sheets are layered together in a stack, optionally with
one or more adhesive materials placed between the sheets to
facilitate binding and lamination of the sheets to each other, and
the stack is laminated into an integral composite membrane under
application of heat and/or pressure. A different approach to making
composite membranes is to cast or form one membrane layer in situ
on top of another layer. The base layer may be a fibrous backing
material or it may be a membrane. The composites may include, for
example, composites having different layers of any of the foregoing
media, composites having multiple layers of the same medium, or
composites having layers of the same medium, but of different pore
sizes, porosities, geometries, orientations, and the like. The
composite may be formed either before or after a membrane component
is coated with a fluorosulfone oligomer according to a preferred
embodiment.
Composite filtration media of the preferred embodiments are not
limited to composites including membranes. Composites including
other filtration media, for example, nonwoven or woven fibers or
any other suitable non-membrane filtration media, are also
contemplated.
In one type of composite, an oleophobic filtration medium of the
preferred embodiment is bonded to a textile fabric or other woven
or nonwoven material to form a layered fabric capable of excluding
the passage of liquid while allowing passage of vapors and gases
therethrough. Such a layered fabric may be preferred in a variety
of applications, as will be appreciated by those of ordinary skill
in the art. Bonding an oleophobic filtration medium, such as a
membrane, to a fabric may be accomplished by conventional
adhesives, thermal bonding, and the like. In this embodiment, the
filtration medium may be coated prior to layering, or the coating
may be applied simultaneously with, during, or after the layering
of the filtration medium with the fabric.
Any polymer capable of being processed into filtration media using
conventional methods, such as, for example, melt-blown techniques,
or that can be formed into a membrane by a casting or other process
and that can be rendered oleophobic by grafting with fluorosulfone
oligomer is generally suitable for use in the present invention.
Generally, oleophobicity is a characteristic of materials
exhibiting repulsion to oils. Oleophobic materials repulse oils and
possess a low surface tension value and are wettable by low surface
tension liquids such as alcohol.
The Fluorosulfone Oligomer
The substrates of a preferred embodiment are rendered oleophobic
through grafting the fluorosulfone oligomer to the polymer of the
substrate such that a covalent bond is formed. Fluorosulfone
oligomers suitable for use in preferred embodiments include those
incorporating a functional group capable of grafting to the polymer
of the substrate. The term `oligomer`, as used herein, is a broad
term and is used in its ordinary sense, including, without
limitation, oligomers incorporating up to about 20 or more repeat
units, for example from about 1, 2, 3, 5, 10, 12, or 15 up to about
20 or more repeat units. For membranes having smaller pore sizes,
oligomers having fewer repeat units are preferred. When the
substrate to be coated is a larger pore membrane, oligomers having
shorter or longer chain lengths may be preferred. Generally, the
larger the pore size of the membrane, the longer the chain length
of the oligomer that may be preferred to coat the membrane without
significant pore blockage. However, longer chain length oligomers
tend to be less reactive than a corresponding oligomer having a
shorter chain length. Thus, if the polymer substrate is resistant
to grafting, a shorter oligomer chain length may be preferred.
More than one fluorosulfone oligomer may be employed
simultaneously. The fluorosulfone oligomers may differ, for
example, in chemistry and/or chain length. The fluorosulfone
oligomer as preferred herein has the ability to cause a surface to
have decreased wettability by a low surface-tension fluid. Wetting,
by definition, is the process of one fluid, including a liquid or a
gas, displacing another fluid at a solid surface. However, in most
cases, the term is used to describe the displacement of air by a
liquid.
Suitable fluorosulfone oligomers may contain chemical functional
groups such as acrylate or methacrylate groups and the like. In a
preferred embodiment, the fluorosulfone oligomer is
polyfluorosulfone acrylate. A nonlimiting example of such a
fluorosulfone oligomer is one having the following structure:
wherein n and m are integers up to about 20 or more, typically from
about 1, 2, 3, 5, 10, 12, or 15 up to about 20 or more, preferably
from about 2, 3, or 5 up to about 10, 12, 15 or 20, more preferably
from about 5 to about 10, 12, 15, or 20, and most preferably
greater than about 10.
The fluorosulfone oligomer is preferably applied to the polymeric
substrate in the form of a solution. Suitable solvents include both
polar and nonpolar solvents, including fluorocarbons, hydrocarbons,
and alcohols such as, for example, isopropanol. Preferably, the
solvent is not a solvent of the substrate. In a preferred
embodiment, a mixture of isopropanol and water is preferred as the
solvent. Nonlimiting examples of other suitable solvents include
t-amyl alcohol, 2-methoxyethanol, ethanol, and methanol. The
solubility of the fluorosulfone oligomer may be limited in certain
solvents, resulting in formation of a suspension or emulsion. It is
also suitable to apply the fluorosulfone oligomer to the polymeric
substrate from such a suspension or emulsion.
The oligomer solution contains sufficient fluorosulfone oligomer to
render the coated membrane sufficiently oleophobic without
substantial pore blockage. The oligomer solution may contain from
about 0.05 wt. % or less to about 40 wt. % or more of the oligomer,
preferably from about 0.1 wt. % to about 10 wt. %, more preferably
from about 0.5 to about 5 wt. %, and most preferably from about 1
to about 2 wt. %. At higher concentrations, substantial blockage of
the membrane's pores may be observed, resulting in lower airflow
through the membrane. At lower concentrations, insufficient
quantities of fluorosulfone oligomer may be available for grafting,
resulting in a coated membrane that is not sufficiently oleophobic.
In either case, the performance of the coated filtration medium may
be less satisfactory, or even unsatisfactory, when compared to that
of a coated filtration medium prepared from a solution in the
preferred range. The optimal concentration of oligomer may vary
depending upon the substrate to be coated. For example, the optimal
concentration of oligomer may be lower for a membrane with smaller
pore sizes and higher for a membrane with larger pore sizes or a
loosely woven substrate.
The solution may also optionally contain one or more grafting
initiators. A suitable grafting initiator is
2-hydroxy-2-methyl-1-phenyl-propan-1-one. The concentration of the
grafting initiator in the solution may be from about 0.05 wt. % to
about 1 wt. %, preferably from about 0.1 wt. % to about 0.5 wt. %,
and more preferably from about 0.1 wt. % to about 0.2 wt. %.
The Grafting Process
The grafting process involves immersing the polymer substrate in
the oligomer solution and allowing the solution to substantially
penetrate the substrate. An immersion time of from about 1, 5, 10
or 15 seconds to about 1, 2, 5 or more minutes is typically
sufficient to allow the oligomer solution to saturate the
substrate. More typically, an immersion time of from about 15
seconds to about 2 minutes is preferred. Immersion times of from
about 30 seconds to one minute are most preferred. Other times may
be advantageous depending on the membrane and the oligomer
formulation. The immersion of the substrate in the solution may be
conducted at any suitable temperature, preferably at ambient
temperature.
After the oligomer is applied to the polymer substrate by
contacting the substrate with, for example, a solution, dispersion
or emulsion of the oligomer, the oligomer is grafted to the
substrate to yield an oleophobic-coated substrate. Grafting may be
initiated by any suitable method. A few nonlimiting examples of
approaches to grafting the fluorosulfone oligomer to a polymeric
substrate include use of a grafting initiator such as
2-hydroxy-2-methyl-l-phenyl-propan-1-one, exposure to ultraviolet
(UV) radiation, or heating to a temperature sufficient to induce
grafting. Suitable temperatures may typically include, for example,
a temperature from just above ambient up to the highest temperature
that the filter is capable of withstanding without sustaining
substantial damage, typically just below the melting point of the
lowest melting component of the filter. A temperature above
100.degree. C. is preferred.
In a preferred embodiment, after contacting the substrate with the
oligomer solution, the substrate is removed from solution and
exposed to radiation to induce grafting of the oligomer to the
polymer of the substrate. Suitable types of radiation include UV
radiation, mixed-color light, infrared radiation, microwave
radiation, or any other radiation capable of inducing grafting. UV
radiation is preferred because it is particularly effective in
inducing grafting. UV radiation has a wavelength of from about 15
nm up to about 400 nm. The wavelength of the UV radiation preferred
for inducing grafting is typical of that of UV-C radiation, which
includes wavelengths of from about 15 nm up to about 280 nm. More
preferably, the wavelength of the UV radiation is about 254 nm.
When grafting is induced by UV radiation, irradiation times
typically will be from about 10 seconds, 15 seconds, 30 seconds, 1
minute, two minutes, five minutes, ten minutes, thirty minutes or
more up to about 1 hour, 2 hours or more. Other times may be
preferred, depending upon the nature of the substrate and oligomer,
as will be appreciated by one of skill in the art. More typically,
the irradiation time is from about one minute up to about one hour,
most typically about 1 hour. The irradiation time may depend upon
the ease or difficulty in inducing grafting of the fluorosulfone
oligomer to the polymer of the substrate. Generally, the more inert
the polymer, the more UV irradiation time is required. The
irradiation may be conducted in an inert atmosphere, such as, for
example, a nitrogen or argon purge.
After the irradiation is completed, the coated substrate is
preferably rinsed to remove residual oligomer. Suitable rinsing
solutions may include water, alcohol, mixtures of water and
alcohol, or any other solvent capable of removing residual oligomer
without causing substantial damage to the coated membrane. It is
also preferable to dry the coated substrate at an elevated
temperature, for example, in an oven. Drying temperatures may be
between a temperature slightly above ambient to any higher
temperature that the coated membrane is capable of withstanding
without substantial damage. For example, drying temperatures of
from about 100.degree. C. to about 150.degree. C. are typically
preferred for coated polysulfone substrates. Alternatively, the
coated substrate may be dried by any other suitable method that
does not substantially affect the performance or integrity of the
coated substrate, such as, for example, air-drying.
The substrate is preferably coated with an amount of fluorosulfone
oligomer sufficient to impart oleophobicity to the coated filter
without substantially affecting airflow through the filter.
The Coated Substrates
The relative oleophobicity of modified filters and unmodified
substrates is determined by testing the filters and substrates
(collectively "filtration media") for their ability to be wetted by
a low surface-tension fluid. A drop of 2-methoxyethanol having a
surface tension of 31.8 dynes/cm.sup.2 at 15.degree. C. is gently
placed on the surface of the filtration medium using a glass
pipette, and the wetting time is recorded. If the medium is not
wetted by the 2-methoxyethanol within 30 seconds, the result is
recorded as "No Wetting". The filtration media of the preferred
embodiments are generally resistant to wetting by 2-methoxyethanol,
and are relatively more oleophobic than untreated substrates.
Airflow through a filtration medium is measured in units of Gurley
Flow. Gurley Flow is the time in seconds it takes 300 ml of air to
pass through a 1" diameter membrane under the force of a 5 oz
weight. Gurley Flow may be measured using a Model 4320 GENUINE
GURLEY.TM. Densometer manufactured by Gurley Precision Instruments
of Troy, N.Y.
A detergent solution penetration test is preferred to determine a
membrane's resistance to penetration by a dilute solution of a
dishwashing detergent. The detergent solution mimics the behavior
of a vitamin solution in contact with a membrane in an intravenous
line. A consumer grade dishwashing detergent, such as Dawn.TM.
available from Proctor & Gamble of Cincinnati, Ohio, is diluted
to produce a 1:100 solution in water. The solution is contacted to
one side of a 25 mm diameter membrane to be tested, and one meter
of head pressure is applied. If no detergent solution penetrates
the membrane after one minute, then the membrane has passed the
detergent flow test.
Water penetration is determined by measuring the pressure in pounds
per square inch differential (psid) required to force water through
the filtration medium. Pounds per square inch differential is the
difference in pressure existing on opposite sides of a filtration
medium. In comparing two filtration media having similar porosity,
the water penetration pressure correlates with the filtration
medium's relative hydrophobicity, wherein a high water penetration
pressure indicates that the filtration medium is more hydrophobic
than a filtration medium having a lower water penetration
pressure.
EXAMPLES
The following examples are provided to illustrate the present
invention. However, such examples are merely illustrative and are
not intended to limit the subject matter of the application.
Example 1
Polysulfone Membrane (BTS-65H) and 1 wt. % Oligomer
A 0.1 .mu.m hydrophobic polysulfone membrane (BTS-65H, 65 psi
bubble point, sold by USF Filtration and Separations Group, San
Diego, Calif.) was rendered oleophobic via grafting to a
fluorosulfone oligomer. An emulsion containing 1 wt. %
fluorosulfone acrylate oligomer (from FluoroChem USA, West
Columbia, S.C.), 45 wt. % isopropyl alcohol, and 54 wt. % deionized
water was prepared. The fluorosulfone oligomer consisted of a
mixture of oligomers of varying number of repeat units having the
formula C.sub.n F.sub.2n+1 SO.sub.2 N(CH.sub.2 CH.sub.2)CH.sub.2
CH.sub.2 OCO--(CH.sub.2 --CH.sub.2).sub.m --CH.dbd.CH.sub.2,
wherein m is between about 2 to 10 and n is between about 5 to
20.
The membrane was immersed in the emulsion for one minute, then
removed from the emulsion and placed in a polyethylene bag which
was purged with argon. The membrane was exposed for one hour to UV
light having a wavelength of 254 nm. The resulting coated membrane
was rinsed with a mixture of isopropyl alcohol and water for ten
minutes, then dried at a temperature of 100.degree. C. for ten
minutes. The coated membrane was tested for water penetration
pressure, detergent solution penetration and airflow. The membrane
modified by grafting with the fluorosulfone acrylate oligomer
displayed a substantially higher water penetration pressure (44
psi) than the untreated membrane (30 psi). Airflow through the
uncoated membrane was measured at 8.5 sec/sq.in/5 oz@10 ml compared
to 9.0 sec/sq.in/5 oz/10 ml for the coated membrane, indicating
that coating the membrane with the fluorosulfone oligomer did not
substantially affect the airflow through the membrane. When an
effect on airflow is observed, it is typically an improvement in
the rate of flow. The coated membrane was subjected to a 1 meter
head pressure detergent solution test. The polyfluorosulfone
acrylate-coated membrane passed the detergent solution test, while
the membrane prior to modification failed the test.
Example 2
Polysulfone Membrane (BTS-45) and 1 wt. % Oligomer
A polysulfone membrane having a bubble point of 45 psi (BTS-45 sold
by USF Filtration and Separations Group) was rendered oleophobic
via grafting to a fluorosulfone oligomer. An emulsion containing 1
wt. % of the fluorosulfone acrylate oligomer of Example 1 in t-amyl
alcohol was prepared. The membrane was coated according to the same
procedure as in Example 1. The coated membrane was tested for water
penetration pressure, detergent solution penetration and airflow.
The membrane modified by grafting with the fluorosulfone acrylate
oligomer displayed a substantially higher water penetration
pressure (37 psi) than the untreated membrane (27 psi). Airflow
through the uncoated membrane was measured at 4.8 sec/sq.in/5 oz@10
ml compared to 5.3 sec/sq.in/5 oz@10 ml for the coated membrane,
indicating that coating the membrane with the fluorosulfone
oligomer did not substantially affect the airflow through the
membrane. When an effect on airflow is observed, it is typically an
improvement in the rate of flow. The coated membrane was subjected
to a 1 meter head pressure detergent solution test. The
polyfluorosulfone acrylate-coated membrane passed the detergent
solution test, while the membrane prior to modification failed the
test.
Example 3
Polysulfone Membrane (BTS-65H) and 10, 15, or 20 wt. % Oligomer
Hydrophobic polysulfone membranes (BTS-65H) were rendered
oleophobic via grafting to a fluorosulfone oligomer. Emulsions
containing 10, 15, and 20 wt. % of the fluorosulfone acrylate
oligomer of Example 1 in isopropyl alcohol were prepared. The
membranes were each immersed in their respective emulsions for one
minute, then removed from the emulsion and placed in a polyethylene
bag which was purged with argon. The membranes were exposed for one
hour to UV light having a wavelength of 254 nm. The resulting
coated membranes were rinsed with a mixture of isopropyl alcohol
and water for fifteen seconds, air dried for 30 minutes, then oven
dried at a temperature of 50.degree. C. for fifteen minutes. Water
penetration pressure was measured for each of the coated membranes,
which were also subjected to the detergent solution penetration
test. Airflow was measured at three different points on each of the
membranes to obtain an average airflow for each coated membrane.
Experimental results are provided in Table 1.
TABLE 1 Water Conc. Air Flow (sec/sq. in/5 oz @ 10 ml) Detergent
Penetration % Point 1 Point 2 Point 3 Average Test (psi) 0 9.0 8.8
7.2 8.5 Fail 30 10 10 8.2 8.1 8.8 Pass >50 15 13.5 5.4 12.7 10.5
Pass >50 20 10.4 7.7 6.5 8.2 Pass >50
The membranes modified by grafting with the fluorosulfone acrylate
oligomer mixtures displayed a substantial increase in water
penetration pressure when compared to the untreated membrane.
Coating the membranes with fluorosulfone oligomer was not observed
to substantially affect the airflow through the membrane. Each of
the coated membranes passed the detergent solution test.
Example 4
PVDF Membrane and 30 wt. % Oligomer
Polyvinylidene difluoride membranes (hydrophilic 0.45 .mu.m pore
size PVDF sold by USF Filtration and Separations Group) were
rendered oleophobic via grafting with the same fluorosulfone
oligomer mixture as in Example 1. An emulsion containing 30 wt. %
fluorosulfone oligomer mixture in isopropyl alcohol was prepared.
The membranes were coated according to the same procedure as in
Example 3. The coated membranes were tested for detergent solution
penetration and airflow. The results of the tests are provided in
Table 2.
TABLE 2 Conc. Air Flow (sec/sq. in/5 oz @ 10 ml) Detergent % Point
1 Point 2 Point 3 Average Test 0 2.2 2.9 2.7 2.6 Fail 0 2.3 2.6 2.9
2.6 Fail 0 2.7 2.9 2.8 2.6 Fail 30 2.7 2.6 2.9 2.7 Pass 30 2.8 2.9
2.7 2.8 Pass 30 2.6 2.9 2.9 2.8 Pass
Airflow through the membranes was not substantially affected by
coating from an emulsion containing 30 wt. % of the fluorosulfone
oligomer mixture. Each of the coated membranes passed the detergent
solution test.
Example 5
Polysulfone Membrane (CVO) and 25 wt. % Oligomer
A polysulfone membrane (1.0 .mu.m pore size CVO sold by USF
Filtration and Separations Group) was rendered oleophobic via
grafting with the same fluorosulfone oligomer mixture as in Example
1. An emulsion containing 25 wt. % fluorosulfone oligomer mixture
in isopropyl alcohol was prepared. The membrane was coated
according to the same procedure as in Example 3. The coated
membrane was tested for detergent solution penetration, water
intrusion and air flow (three separate times at three points on the
membrane) immediately after the coating treatment. Airflow was also
measured after 12 hours, 48 hours and 3 weeks. Between airflow
measurements, the coated membrane was stored at room temperature.
The results of the tests are provided in Table 3.
TABLE 3 Water Deter- Air Flow (sec/sq. in/5 oz @ 300 ml) Penetra-
gent Point 1 Point 2 Point 3 Average tion Test Before 26.7 28.3
25.6 26.9 7 psi Fail Treatment Immediately 31.2 29.3 29.2 29.9 11.5
psi Pass After 30.2 29.6 30.6 Treatment 29.6 29.8 29.8 12 Hrs.
After 30.4 27.4 30.4 29.6 not not Treatment 32.7 27.4 29.8 done
done 29.8 29.8 28.6 48 Hrs. After 30.5 29.2 30 30.0 not not
Treatment 31 29.5 29 done done 30.9 29.4 29.2 3 Weeks After 30.2
29.5 30 29.9 not not Treatment 32.3 29.3 30.1 done done 29.8 29.2
29
Airflow through the membrane was not substantially affected by
coating from an emulsion containing 25 wt. % of the fluorosulfone
oligomer mixture. Airflow was not affected by the passage of times
up to three weeks from the initial coating treatment. Water
penetration increased significantly after coating. The coated
membrane passed the detergent solution test.
The present invention has been described in connection with
specific embodiments thereof. It will be understood that it is
capable of further modification, and this application is intended
to cover any variations, uses, or adaptations of the invention
following, in general, the principles of the invention and
including such departures from the present disclosure as come
within known or customary practices in the art to which the
invention pertains and as may be applied to the essential features
hereinbefore set forth, and as fall within the scope of the
invention and any equivalents thereof.
* * * * *